Homing supercavitating torpedo concepts currently being considered in ongoing research and development programs employ cavitators, which may have a positive pressure drag coefficient, to produce a cavity that expands outward in a downstream direction from the cavity inception point to some maximum cavity radius, then contracts to the point of cavity closure, usually positioned downstream of the body. A significant drag advantage can be obtained via the near elimination of friction drag.
However, the small wetted area of the cavitator poses problems if that area is to host transducers serving as forming the elements of a sonar system. Specifically, the amount of acoustical power that can be transmitted via the small wetted area is limited: overpowering the system causes cavitation on the nominally wetted transducer faces, causing severe performance degradation. Furthermore, the aperture of the sonar array is limited by the small cavitator diameter, and the number of array elements that can be practically packed within such a small volume is also quite limited, which in turn limits the beam-forming capabilities of the system. Since drag of such a cavitator is directly proportional to its projected area at the locus of cavity detachment, simply increasing the cavitator size is not a practical option, since it would eliminate the drag advantage that is otherwise gained via supercavitation. In order to provide enough elements in this small space to form beams to localize a target with respect to the array, the elements must be relatively small, and therefore must operate at higher acoustical frequencies than those on a conventional torpedo array.
Sound navigation and ranging (SONAR) is a technique that uses sound propagation under water to navigate or to detect objects in or on the water. As is well known in the relevant art, there are two types of sonar: passive and active. Passive sonar seeks to detect an object target by listening for the sound emanating from the object being sought. Active sonar creates a pulse of sound, and then listens for reflections of the pulse from a target object. To determine the distance to the target, elapsed time from emission of a pulse to reception is measured. To determine the directional bearing, several hydrophones are used to measure the relative arrival time to each in a process called beam-forming.
Conventionally, most active sonar systems cannot receive while they are transmitting. This is because conventional active sonar systems use the same device, (called a ‘transducer’), to both transmit and receive, and transducers cannot both transmit and receive at the same time.
Conventional art thus fails to show how to effectively discriminate between signals that sonar receives from its own transmitter and echoes from the intended target subject to certain linearity and rejection requirements.